For sketching the stone is painted opaque white.
Ready, steady, go.
By the way, off-the-shelf milling cutters, used by goldsmiths and in dental labs, do not run smoothly enough for the purpose. So engravers actually produce their tools themselves from tool-steel blanks.
Engravers need steady hands as the whole process is performed freehandedly.
The actual cutting is achieved by diamond powder mixed with oil, applied by means of a quill.
For faster cutting, more diamond is required, applied by the mortar used to grind the diamond powder.
Almost done.
Final polish.
Imprint in sealing wax.
Ready for shipping.